Transition device



Feb. 27, 1962 H. J. ROWLAND TRANSITION DEVICE 5 Sheets-Sheet 1 Filed Sept. 8. 1959 Feb. 27, 1962 H. J. ROWLAND TRANSITIONDEVICE 5 Sheets-Sheet 2 Filed Sept. 8, 1959 +IML1L I i o MIU I ti 2'34 llll Inveai oa- .l'fizwaifdJfBowiazzcl fizz, W24 1;

Feb. 27, 1962 H. J. ROWLAND TRANSITION DEVICE 3 Sheets-Sheet 3 Filed Sept. 8. 1959 Izw 3 Howard JRowZand, fife/T (3W M 'y 3,023,381 Patented Feb. 27, 1962 fiice chusetts Filed Sept. 8, 1959, Ser. No. 838,761 13 Claims. (Cl. 333-33) This invention relates to guided electromagnetic wave transmission and more particularly to a broadband coupling system between coaxial transmission line and waveguide.

In transmission lines used at very high and microwave frequencies it is frequently desirable and often necessary to change from a coaxial transmission line to a waveguide. To give but one example, many oscillators operating in the general range of to 10 cycles utilize coaxial output terminals which are worked into the more efiicient waveguide system. Generally, a transition is desired in which the characteristic impedance of the coaxial line and the waveguide are fairly matched over the useful bandwidth of the system, while at the same time discontinuities arising from geometry of the transitions are minimized. A good match provides the most eflicient transfer of wave energy from one system to the other. To express these ideas in different terms, what is wanted is a transition system which keeps the Voltage Standing Wave Ratio (V.S.W.R.) at a fairly constant minimum over the desired bandwidth in both waveguide and coaxial line.

Over the past two decades, when activity in this art has been intensified, a good many constructions have been offered which purport to provide satisfactory transition. Most of the successful and commercially feasible devices, however, have served adequately only in situations which call for comparatively narrow bandwidth requirements, for example, 10% at some particular minimum V.S.W.R., usually 1.1. This 10% bandwidth is of course a good deal less than the usable range of most waveguides. For a good many years, therefore, considerable effort has been made in attempts to make the bandwidth characteristics of transition devices more nearly comparable to the useful bandwidth of the associated waveguide, which range from 25 to 40%. Generally, the starting point has been one of modifying successful narrowband transitions.

Amongst many, the most common and simplest of the narrowband devices is the configuration which causes the inner conductor of the coaxial line to enter the waveguide as a probe. This so-called resonant stub arrangement is critical as to length of probe, distance of probe from the guide termination, and probe thickness. However, it is rather a simple matter to tune such a device for exact match of the transition discontinuity at a single wavelength.

The more diflieult problem is the design of broadband transition devices. Here too, a good many constructions have been worked out but only a few have proved to be at all feasible. One design in use currently is a so-called door-knob termination of the coaxial line inside the waveguide. This exhibits fair matching characteristics over a band of frequencies in the order of 10% to and is capable of very high power handling capacity. However, a crucial defect in this configuration is the extremely high precision required in manufacture and assembly, with mechanical misalignment after assembly a further limiting factor on the usefulness of the device.

Another form of broad band transition which was adopted at an early stage of the art is the termination of the inner conductor of the coaxial transmission line in a cylindrical member comparable in diameter to the inner conductor, inside the waveguide normal to the inner conductor and attached to opposite narrow faces of the waveguide. This construction permits a low V.S.W.R., in the order of 1.1 over a frequency band of about 15% maximum. It is easy to construct, does not require special assembly techniques, and, in comparison with doorknob types of transition, is relatively uninfluenced by rough handling or use. Furthermore, it provides an effective heat sink to improve power dissipation characteristics. But in some applications its virtues were insufiicient-especially where rather broad band pass characteristics, of more than the maximum attainable 15% were imperative.

One solution was to return to the simple narrowband probe inserted part way into the waveguide using a Teflon or similar insulation material which significantly broadened band width characteristics, albeit at a small sacrifice through insulation losses. But even Teflon was subject to serious breakdown characteristics and deterioration when used in high power applications, particularly highpower non-pulse continuous-wave, very high frequency and microwave transmission.

An object of this invention, accordingly, is to provide a transition device exhibiting wide band width characteristics in the order of 30% with a very low V.S.W.R. over the whole band. Another object of the invention is to achieve wide bandwidth without any sacrifice of power handling capability. A further object of the invention is to make the use of insulation material unnecessary. Yet, another object of the invention is to produce a device having all the foregoing characteristics in a simple, rugged, compact structure.

In the accomplishment of these and other objects of my invention, I provide a terminated or shorted section of rectangular waveguide having an opening in one of the broad faces with an attached coupling unit or flange concentric with the opening for an associated coaxial line. Directly underneath the coaxial line coupling flange, and interior of the waveguide, is a flat bar, parallel to the large waveguide dimension or broad faces attaching to two smaller or narrow faces. The inner conductor of an associated coaxial line protrudes beyond the outer conductor coupling flange and fits into a cylindrical sleeve mounted concentrically in the flat bar which extends above and below the broad plane of the flat bar.

A feature of this present invention achieved by the use of the fiat bar, is that the useful bandwidth is increased by a factor of more than three times over that of the conventional cylindrical bar-from about 5 to 10% to approximately 30% at any V.S.W.R. Another feature of the invention is that the significant increase in bandwidth is achieved without the need for insulation material so that high power is not limited by some extraneous unpredictable material. A further feature of the 1 invention is that the flat bar provides an effective heat sink facilitating high power dissipation. Still another feature is the relative ease of construction. The flat bar is simply placed into slots cut in the narrow faces of the waveguide and then welded.

These and other objects and features of the invention will be more apparent as the description proceeds with the aid of the accompanying drawings in which:

FIG. 1 is a perspective view of a transition device at the junction of the coaxial transmission line and the waveguide, partially cut away to show the termination of the coaxial inner conductor in a flat bar, interior of the waveguide.

FIG. 2 is an end view of the transition device looking from the waveguide system end into the transition system.

FIG. 3 is a cross-section of the transition device taken along the line 33 of FIG. 4.

FIG. 4 is a top view of the transition device.

FIG. 5 is a side view of the transition device.

FIG. 6 is a detail of a variation of the sleeve mounted in the flat bar.

The description which follows is divided into two parts, the first deals generally with the completed transition device, the second deals generally with the construction and assembly thereof.

Referring now to the drawings, FIG. 1 shows a rectangular hollow waveguide section 2 joined to a coaxial transmission line generally designated at 4, forming a completed transition device or coupling system for the transfer of electromagnetic energy in either direction. The waveguide section 2 has broad faces 6 and narrow faces 8 as can best be seen in FIGS. 1, 4 and and the associated coaxial line 4 comprises an outer conductor 10 and an inner conductor 12 which extends beyond the end of the outer conductor. In one of the broad faces 6 there is an opening 14 of about the same diameter as that of the outer coaxial conductor 10 and centered where the median line of the waveguide 2 intersects the axis of the coaxial conductor 4.

In most transition devices embodying the invention a coupling flange 16 is mounted on a short cylindrical pipe section or pedestal 18 having substantially the same dimensions as the outer conductor 10 of the associated coaxial line 4. The flange 16 is generally welded to the pedestal 18 and is designed to fit closely to a similar flange 20 secured to the outer conductor 10 of the associated coaxial transmission line 4 as indicated in FIG. 1.

Inside the waveguide section 2 and beneath the pedestal 18 a flat bar 22 is disposed so that its broad surfaces 24 are parallel to the broad faces 6 of the waveguide. As will be explained in more detail later, the ends of the bar 22 are fitted into slots 23 in the narrow faces 3 of the guide and then welded from the outside.

In FIGURES l-5 there will be seen a sleeve 26 inserted at right angles into an appropriate aperture in the broad surfaces 24 of the flat bar 22. This sleeve 26 in one embodiment has a shape best described as a cylinder, the axis of which is identical with the axes of the cylindrical pedestal 18 and the outer conductor 10 of the coaxial transmission line 4. The interior diameter of the portion of sleeve 26 adjacent to the pedestal 18 is selected to receive snugly the inner conductor 12 of the coaxial line 4 which conductor extends beyond the flange 20 of the outer conductor 10 and terminates inside the sleeve 26.

A partition 27 with a threaded hole therein divides the sleeve 26 into two sections. As can be seen in FIGS. 3 and 5, the hole in the partition 27 receives the threaded shank end 29 of a rounded probe plug 28 which fits the inside of that part of the sleeve 26 beneath the flat bar 22.

As will be explained more fully in the second part of the description, the probe-plug 28 is left adjustable on each production unit until electrical tests are made. When the optimum position is determined, the plug 28 is welded to the sleeve 26. A gas inlet hole 30 in one of the broad faces of the guide is conveniently located for adjusting the plug prior to welding by inserting a screwdriver in the indent 31 of the plug 28 through the hole 30. This hole 30 is closed by inserting a simple threaded screw (not shown) after the system is assembled and filled with gas.

Referring now to the terminated end 32 of the waveguide section 2 of the transition device, in most applications this end 32 is made of the same material as the guide section 2 itself. While in certain sizes of the transition device it may be feasible to integrally cast the end piece 32 with the rest, in most sizes it is easier to weld the end piece 32 after the flat bar 22 and cylindrical sleeve element 26 are assembled with the waveguide.

The opposite, open end of the waveguide section is provided with a standard flange 34 which might also be integrally cast but which is generally a separate piece welded to waveguide section 2. FIG. 1 shows the flange 34 attached to a similar flange 36 connecting to a waveguide system (not shown). In FIG. 2 the dimensions of the flange 34 can be readily compared to both the general size of the coaxial flange l6 and the waveguide 2 dimensions.

It is well-known to those skilled in the waveguide art that the mere configuration of a device is no more than a starting point for the often laborious task of adjusting many interrelated dimensional parameters, particularly with a device such as that described herein. This part of the description, therefore, will be addressed to the construction of the transition unit, the dimensions which have been worked out to achieve wide bandwidth, and the electrical testing of production units.

While the foregoing description applies to all sizes of transition devices, from the huge units over a yard wide used at frequencies as low as 300 mc., to the handsize units at 10,000 mc., the sizes do differ in certain minor respects. Such differences as exist, however, are a matter of convenience of construction and assembly rather than any departure from the spirit of the invention. In the smaller units used at higher frequencies, a standard twenty foot length waveguide is cut into appropriate lengths to form the waveguide segment 2. These twenty foot lengths are usually the standard RETMA extruded waveguide. In the larger sizes, the waveguide segment 2 is fashioned from four pieces of appropriate sheet stock to the required dimensions. In both cases, the edges of the segment 2 thereformed are carefully finished to precise dimensions. Some success has attended eflorts to cast waveguide segment 2 in the smaller sizes and it is expected that as the technique of casting is improved, this will replace the use of ordinary RETMA extruded waveguide cut into appropriate lengths.

The next operation involves the milling or cutting out of the opening 24 upon which the pipe or pedestal 18 will seat on the broad face 6 of the guide segment 2. The somewhat oversize slots 23 into which the flat bar 22 will later be fitted are cut at the same time. Since the pedestal 18 has already been welded to the coaxial flange 16 in a separate operation, that unit may now be welded in place over the milled out aperture in the broad face 6 of the segment 2. The standard RETMA waveguide flange 34 is welded to the end of the segment 2 at the same time.

In another series of operations the flat bar unit comprising the flat bar 22 itself, the cylindrical member 26 and the adjustable probe plug 28 are assembled. The flat bar 22 is metallic, usually of the same material as the waveguide segment 2 and is milled and finished to specification. An aperture is then drilled out to receive the cylindrical member 26.

As will be best seen in FIG. 3, this cylindrical member 26 in the smaller transitions is usually formed from solid cylindrical stock drilled out at each end leaving a partition 27 closer to one end of the member 26 than the other. This partition 27 as previously explained has a small hole drilled and threaded into it to receive the shank 29 of the probe plug 23. The cylindrical member 26 is then set into the flat bar aperture and welded thereto about its circumference, the weld being smoothed off carefully to form a continuous smooth surface from flat bar 22 to the cylinder 26. The plug 28 is now inserted into the cylinder 26, the threaded shank end 29' screwed into the threaded hole in the partition 27 of the cylindrical member.

In the smallerunits it is convenient to place the standard gas hole 36 directly beneath the probe plug 28 permitting ready access to' the plug for adjustment during electrical testing. This is not necessary in the larger guide since it is quite simple to adjust the plug 28 by hand. The gas hole or outlet 30 is a standard feature of waveguide systems for the introduction of gas into the waveguide and is closed by a plug or threaded screw (not shown).

In the larger size transitions instead of drilling out solid cylindrical stock to make the cylindrical member 26, standard tubular stock is cut to size. One end receives the inner conductor 12 of the coaxial line 4, the other end receiving the probe plug 28. In this case the probe plug 28 is of a size to fit snugly inside the cylindrical member and does not require the pierced partition and threaded shank shown in FIG. 5. This frictionally held plug is moved up and down by hand manipulation when electrical tests are made. Attention is called to a variation of the sleeve just described as shown in FIG. 6. Occasionally, it becomes convenient to utilize the same waveguide with a smaller coaxial conductor. In such a case that part 40 of the cylindrical tube 42 which receives the inner conductor of the coaxial line is reduced in diameter to make the necessary snug fit therewith. A trunoated conical section 44 connected at the flat bar connects to the probe plug end 46 of the tube which remains the same size.

The unit consisting of the flat bar 22, cylinder 26, and probe plug 28 is then ready to be assembled with the body of the transition. This is done by fitting the ends of bar 22 into the already cut slots 23 provided in the narrow faces 8 of the guide 2. Welding of the fiat bar 22 to the guide 2 is done from the outside of the narrow faces 8 while holding the bar 22 toward the top and rear of the slots 23, i.e., away from the waveguide flange 34 towards the end 32 which will later be closed. Combining the flat bar unit with the waveguide unit is considerably easier in the larger transitions because the dimensions permit a man to get his hands inside the waveguide to position the fiat bar in the appropriate slots out therefor. Furthermore, in some of the largest units, slots are not required for the fitting of the flat bar; the flat bar equal in length to the interior transverse dimension of the segment 2 need merely be placed in position and welded from the inside. Once the fiat bar unit is welded in place, the end of the waveguide segment 2 which is to be closed has the end plate 32 welded on, the weld being champfered on the outside. The end plate 3-2 is, of course, metallic and is generally of the same stock although sometimes of less thickness than the material from which the waveguide itself is made.

The last step prior to electrical testing consists of temporarily welding the irises 48 to the inside of the narrow faces 8 of the guide 2. The same temporary welding or tacking, as it is called, is performed when a tuning stub 50 is used, either instead of the irises 48 or together therewith. The use of tuning irises and stubsis determined by the specifications to which the transition system must conform. Some transition devices do not require such tuning elements to achieve match at low VSWR, but in general it is found most convenient to facilitate production by use of these simple tuning elements to compensate for the inevitable idiosyncrasies present in each individual unit; where, for example, a drop of weld material in an obscure corner of a transition unit can appreciably alter its electrical characteristics. In the laboratory careful control may be kept over tolerances in construction of experimental and prototype transition devices but the same care may not be feasible where many units must be turned out in production, assembled, tested, packed and shipped within a short period of time.

The practically complete transition unit is ready for electrical testing after the tacking of the tuning elements 48 and 50. Plug 28 is still adjustable and this is employed to make certain that the individual units conform to specifications. Often, the adjustments are simple and the plug 28 can then be welded finally along with the irises 48 or tuning stub 50. Sometimes, however, the adjustment of the probe plug 28 is insufficient to bring the unit within specifications and the irises must be untacked as it were, and repositioned. Under normal circumstances, where a production run is being made, only the first few units require retacking of the tuning irises 48 and stub 50. The individual peculiarities of men and machinery in the assembly and welding processes are then compensated for by slight changes in tacking positions for the irises and stubs. Afterward only the probe plug 28 itself need be adjustable to secure the expected performance of the unit.

It was found that the experimental results for one size waveguide and coaxial transmission line held fairly constant when the results were scaled both upward and downward. But the convenience of scaling results and dimensions worked out at one waveguide dimension and one coaxial transmission line dimension to larger or smaller components is, of course, limited by the availability of standard waveguide and coaxial sizes. In order to accommodate the greatest interchangeability of components, only standard RETMA waveguide and coaxial sizes have been employed. Therefore the Smith chart technique is required to maximize matching and minimize VSWR for any combination of various components in one particular transition device.

Table I [Dimensions are in inches] A B O D F 1 G A 5. A A A 1 Has no generalized electrical significance. Need only be long enough to properly position irises.

Table II Size A B O D F G (approx) The tables set out above give a representative selection of dimensions for various size transition networks. Table 1 sets out the relationship between the various dimensions shown in FIG. 5 in relation to the free-space cutoif wave length. This information should be of considerable assistance in designing waveguide transition devices of all sizes. As already noted, the experimental results for one size waveguide and coaxial transmission line hold fairly constant when the results were scaled both upward and downward. The variations for broadband pass at a low VSWR from the relationships of Table II varied in ar range from 5 to 15%, variations which are known to those skilled in this art to be extremely small.

Certain minor variations of the preferred embodiment of my invention will be apparent to those skilled in the art. For instance, the foregoing description of a transition system was in terms of a device which is constructed as a distinct unit tobe connected to appropriate flanges or connector elements on a waveguide and coaxial line respectively. The spirit of the invention, however, contemplates more generally the termination of the inner conductor of a coaxial transmission line in a flat bar, interior of a waveguide, and this clearly can be integrally constructed for a given system.

Therefore it is not my intention to confine my invention to the precise form herein described but rather to limit it in terms of the appended claims.

I claim:

1. A transition system for electromagnetic energy corm prising a coaxial conductor transmission line having an outer conductor surrounding an inner conductor, a waveguide having broad faces and narrow faces, said inner conductor extending into said waveguide through an opening in a broad face of said Waveguide and forming a probe therein, said outer conductor being connected to said broad face, said probe touching and protruding at right angles through a substantially flat cross-bar being parallel to said broad face, and connected to said narrow faces, and a short circuit termination across said waveguide spaced from said coaxial line.

2. A coupling system as set forth in claim 1 the axis of said probe being appreciably disposed away from the median longitudinal axis of said flat cross-bar.

3. A coupling system as set forth in claim 1 wherein the distance from the axis of said probe to said waveguide end is approximately the distance between the top surface of said flat bar and said broad face is approximately edge of said flat bar nearest the closed end of said waveguide is approximately and the width of said flat bar is approximately where x is the free space cutoff wavelength of the waveguide.

4. In a Waveguide to coaxial line transition device, a section of rectangular waveguide having broad faces and narrow faces, means for coupling the outer conductor of a coaxial transmission line to one broad face of said waveguide, an opening in said broad face concentric with the outer conductor of said coaxial transmission line through which the inner conductor of said coaxial line protrudes into the interior of said waveguide, said inner conductor having an end forming a probe therein, a flat cross-bar generally parallel to said broad faces and connected to said waveguide narrow faces, walls forming an aperture in said fiat bar through which the said probe may protrude and make connection therewith, and a short circuit termination across said waveguide spaced from said coaxial line.

The transition device set forth in claim 4 wherein the folowing approximate relationship exists amongst various elements,

where A equals the distance between the closed waveguide end and the axis of the probe, B equals the distance between the top of the flat cross-bar and the inside surface of the broad face of the waveguide, where C equals the distance between the edge :of the fiat cross-bar closest to the closed end of the waveguide and the axis of the probe, and D equals the Width of the cross-bar, where )r equals the free space cutoii wavelength of the waveguide.

6. In a broadband waveguide transition to coaxial transmission line device, a rectangular waveguide section with broad faces and narrow faces, one end of said wave guide being closed, an opening through one of said broad faces, a coaxial conductor coupling means attached to one broad face of said waveguide section at said opening for connecting to the outer conductor of an associated coaxial transmission line, a flat cross-bar element interior of said waveguide mounted parallel to said broad faces and attached to the narrow faces thereof near the coaxial coupling means, a probe element at right angles to said flat cross-bar extending through and below and above the fiat cross-bar, the axis of said probe substantially coinciding with the axis of said coaxial coupling means, and means for connecting the inner conductor of the said associated coaxial line to said probe element.

7. The device set forth in claim 6 said probe element consisting of a cylindrical member extending at right angles through said fiat cross-bar, the end above the said bar adjacent to the coaxial transmission line coupling means being adapted to receive the inner conductor of said coaxial line.

8. The'device set forth in claim 7 wherein the end of the said cylindrical probe below the said flat cross-bar terminates in a plug, one end of said plug being adapted to fit snugly into said lower end of said cylindrical probe, the exposed end of said plug being rounded.

9. The device set forth in claim 8 including means whereby the said plug may be manually adjusted in an axial direction.

10. The device set forth in claim 7 whereby the said cylindrical probe is in part tapered, the portion of said probe beneath the said flat bar being larger than the end above said flat cross-bar, said end adapted to receive the end of the said coaxial inner conductor.

11. A transition device as set forth in claim 7 including means opposite the said closed waveguide end for coupling said waveguide section to an associated waveguide system.

12. A transition device as set forth in claim 11 said means comprising a flange whereby coupling may be effected to a similar flange on an associated waveguide system.

13. A transition device as set forth in claim 12 wherein the distance from the axis of said probe to said waveguide end is approximately the distance between the top surface of said flat bar and said broad face is approximately the distance between the closed end of said guide and the edge of said fiat bar nearest the closed end of said waveguide is approximately ext-M? References Cited in the file of this patent UNITED STATES PATENTS 2,619,539 Fano Nov. 25, 1952 2,829,348 Kostriza et al. Apr. 1, '1958 FOREIGN PATENTS 781,882 Great Britain Aug. 28, 1957 OTHER REFERENCES Ragan: Microwave Transmission Circuits, Rad. Lab. Series, McGraw-Hill, 1948, pages 340 and 346-349. 

